With a global warming potential (GWP) 310 times greater than CO2, N2O is an extremely potent greenhouse gas (GHG). Models of various emission scenarios worldwide published by the IPCC have suggested a steady increase in N2O production through the 21st century. The impact of such great levels of N2O would result in a significant increase in atmospheric heat retention.
In addition to N2O, other forms of reactive nitrogen also pose a great threat to the environment. Human alteration of the nitrogen cycle via the Haber process, intensive crop cultivation, and fossil fuel use has approximately doubled the rate of nitrogen input to the terrestrial nitrogen cycle. Loss of this anthropogenic nitrogen to natural systems has led to an array of environmental and public health problems, including ammonia toxicity to aquatic life, eutrophication of nutrient limited natural water bodies, oxygen depletion, and vast dead zones in the ocean margins. It is thus apparent that approaches to N2O mitigation must be accompanied by strategies to control reactive nitrogen to natural environments.
The traditional objective of wastewater treatment is to achieve complete conversion of nitrogen compounds in waste to N2 gas. This is accomplished by oxidizing the nitrogen to nitrate then reducing the nitrate to N2. N2O gas is not deliberately produced, but is often incidentally generated at levels that are low but still problematic for greenhouse gas emissions. Due to its negative environmental effects, researchers have never attempted to maximize N2O production rates. To the contrary, researchers have instead focused on minimizing or eliminating N2O production in these processes.
Domestic wastewater contains organics and reduced forms of nitrogen (organic N and ammonia) present as soluble and particulate forms and at relatively low concentrations. For the biodegradable organic matter, energy is often recovered as methane using anaerobic consortia of bacteria and archaea. These microorganisms oxidize waste organics, releasing the electrons and hydrogen as methane gas. Bioreactors designed for methane fermentation are common throughout the world, with applications that range in scale from simple low-rate household systems to sophisticated high rate industrial processes. The majority of these anaerobic bioreactors are “digesters”, because they have as a major design objective the reduction and stabilization of biomass for disposal.
Bioreactors are also used for nitrogen removal. Their function is to accelerate different steps in the nitrogen cycle, so as to prevent the harmful effects of N discharge: ammonia toxicity to fish, eutrophication, nitrate harm to infants, and dissolved oxygen depletion. In conventional systems, nitrogen is processed as shown in FIG. 3A. Ammonia is oxidized to nitrate, a two-step process termed nitrification that requires 2 moles of O2 per mole of N. This oxygen is added by aeration, a process that constitutes about half of the operating expense of a wastewater treatment plant. The rate-limiting step in nitrification, the oxidation of ammonia to nitrite, is catalyzed by two distinct groups of microbes—the ammonia-oxidizing bacteria (AOB) and the newly discovered ammonia-oxidizing archaea (AOA). Most nitrite is then oxidized to nitrate by several distinct groups of nitrite-oxidizing bacteria (NOB), but under some conditions, particularly under low O2 concentrations, AOB (and possibly AOA) emit N2O in a nitrite reduction process termed nitrifier-denitrification. Nitrate nitrogen resulting from nitrite oxidation may then be denitrified to N2, a step requiring 5 moles of electrons per mole of N. In conventional systems, the electrons needed for denitrification come from organic matter, decreasing the number of electrons that can be routed to methane production. Denitrification also results in the production of large quantities of waste microbial biomass for disposal.
Over the last decade, innovations in N removal (i.e., the SHARON, OLAND, use of anammox bacteria, CANON processes) have occurred in European labs. These innovations exploit new understanding of microbial ecology so as to “short-circuit” the nitrogen cycle. The result is a significant decrease in the requirements for O2 and reducing power. An example is the CANON process (Completely Autotrophic Nitrification Over Nitrite) illustrated in FIG. 3B. In this process, partial oxidation of ammonium to nitrite by AOB under bioreactor conditions that select against NOB is coupled to the anaerobic oxidation of ammonium to N2 by anammox bacteria. The anammox bacteria convert nitrite and ammonium to N2 gas through a hydrazine intermediate that apparently avoids N2O production. In principle, this process can achieve a 62% decrease in oxygen and a 90% savings in reducing power, but it is handicapped by the slow growth rates of the anammox bacteria, with doubling times on the order of 10-12 days. Other such innovations can dramatically alter the energy budget for wastewater treatment both by decreasing the energy invested for aeration and increasing the energy recovered as methane. As yet, however, no method of nitrogen removal enables direct energy extraction from the waste nitrogen itself.